Only the Crown post offices are on strike - those are the large post offices in some larger towns and cities. It is only 3 per cent of post offices, so you'll still find 11,000 of them open..............on Tuesday! You may be able to find one that is open though, eg Bourton-on-the-water in the Cotswolds is probably open. Check you local offices on the website http://www.postoffice.co.uk/branch-finder
If you're in or near Scotland ours are open (apart from a dozen or so closed by strike action).
You can buy our funny foreign stamps when you're here. :)
Some village shop post offices have started opening all the hours the shop is open - whether it extends to bank holidays I don't know. Certainly open Sundays